Sensors that can directly measure curvature are desirable for a number of reasons. In applications where the sensor is embedded within the structure, the precise location of the sensor need not be known in order to determine the curvature. This is important for composite materials in which the sensor location may change during the cure process. Curvature sensors can also be located along the neutral axis of the structure where the bending strain is zero and hence is not a suitable location for strain gauges. A direct measurement of curvature is increasingly important for thin structures where the bending strain is considerably reduced.
Previous methods of shape measurement using fiber-optic sensors (e.g., U.S. Patent Application Publication application Ser. No. 20030072515 to G. H. Ames et al., incorporated herein by reference) have used separate Bragg grating strain sensors bonded to opposing sides of a structure that is subject to bending. The differential strain measured by the fiber Bragg gratings (“FBGs”) yields the curvature of the structure. However, the mechanical arrangement is critical to the accuracy of the measurement. The accuracy is dependent on good strain transfer between the host structure, the fiber buffer and the optical fiber. Care must also be taken to avoid temperature gradients across the structure which would lead to erroneous bend measurements. Inscribing Bragg grating strain sensors into separate cores of a multicore fiber (“MCF”) and measuring the differential strain between cores yields a greatly improved sensor. The fused silica structure provides excellent mechanical stability and the core spacing is very stable. Also, due to the close proximity of the FBGs (typically 50-100 microns), the sensitivity to temperature gradients is greatly reduced and temperature independent measurement of curvature is possible, such as discussed in Flockhart et al., “Two-axis bend measurement with Bragg gratings in multicore optical fiber,” Opt. Lett. 2003, 28 (6), pp. 387-389, incorporated herein by reference. However, the close proximity of the cores also reduces the curvature sensitivity (the differential strain between two cores in the MCF subjected to a curvature of 1/R, where R it the radius of curvature, is proportional to their physical separation). Small optical fiber diameters are desirable for smart structure applications where the optical fiber is embedded into the structure. Thus, to increase the response to bending, the cores of the MCF can be configured to be Fabry-Perot cavities. This has been described in, for example, U.S. Pat. No. 6,301,420 to Greenway et al., incorporated herein by reference, and U.S. Pat. No. 6,389,187 to Greenway et al., incorporated herein by reference.
Another MCF sensor has also been described in U.S. Pat. No. 5,563,967 to J. Haake, incorporated herein by reference. This sensor comprises two cores in which fiber Bragg grating strain sensors are formed. Each sensor comprises two FBGs, one in each core. Their Bragg wavelengths are made largely different such that their response to temperature and strain is different. This permits both temperature and strain to be measured and hence provide a measure of one independent of the other.